The present invention relates to fluid catalytic cracking of hydrocarbons. More particularly it relates to an improved method of dispersing feed of liquid hydrocarbons into a stream of heated catalyst particles in a riser reactor utilizing an improved fluidized catalytic cracking nozzle assembly to promote catalytic action between the hot catalyst particle surfaces and finely divided liquid drops.
Fluidized catalytic cracking of heavy petroleum fractions is one of the major refining methods to convert crude or partially refined petroleum oil to useful products, such as fuels for internal combustions engines and heating oils. In such fluidized catalytic cracking (FCC), high molecular weight hydrocarbon liquids are contacted with hot, finely divided, solid, catalyst particles in an elongated riser or transfer line reactor. The reactor is usually in the form of a riser tube and the contact time of the material is on the order of a fraction of a second to a few seconds, say one to ten seconds, and generally not over about three seconds. This short contact time is necessary to optimize generation of gasoline and middle distillate fractions. By proper selection of temperatures and reaction times a catalytic cracking reaction is "quenched" so that economically undesirable end products of such a reaction, methane and carbon, are held to a minimum, and yield of desired products, gasoline, and middle distillate oils, is at a maximum. During this short reaction period a hydrocarbon feedstock, frequently in the form of vacuum gas oil, cycle oil, or the like, at an initial temperature of from about 300.degree. F. to 800.degree. F., is sprayed onto catalyst at temperatures in the range of about 1100.degree. F. to 1400.degree. F. The present invention, as noted above is particularly directed to a method and a nozzle assembly having a nozzle unit and a protective pipe for uniformly misting such feed onto the hot catalyst.
Generally the mixture is fluidized partially by steam, but primarily by hydrocarbon gases that evolve by the hydrocarbonaceous feed vaporizing upon contact with the hot catalyst. The reaction of the mixture is one of essentially instantaneous generation of large volumes of gaseous hydrocarbons. The hydrocarbon vapors and catalyst mixture flow out of the riser tube into a separator or disengaging vessel. The spent catalyst is separated primarily by gravity and inertia forces acting on the catalyst in the separator vessel, and passed downwardly through a stripper section for return to a regenerator. Steam also generally flows up through the down-flowing catalyst to assist in stripping hydrocarbon vapor from the spent catalyst. Heat for the process is obtained by burning the coke, primarily carbon, on the spent catalyst by flowing oxygen through a bed of spent catalyst in a regenerator vessel. The regenerated and heated catalyst is then recirculated to the riser reactor. The desired product, hydrocarbon vapor, is recovered overhead from the separator vessel. Generally, this recovery is through one or more cyclone separators connected to a plenum chamber or common piping and directly piped to a distillation column. Vapor flow through the cyclone separators extracts residual or entrained catalyst fines. The catalyst fines are recovered from the cyclone separators through "dip legs" connected to the spent catalyst stripper at the bottom or below the disengaging vessel for return to the generator.
A particular problem in the initial generation of hydrocarbon vapor is that if the hydrocarbon liquid does not directly contact catalyst upon injection into the reactor riser, thermal cracking appears to be favored over the catalytic reaction. Such thermal cracking tends to generate end-products of methane and coke. That is, complete conversion of hydrocarbons in the feed produces gas and coke, rather than desired middle distillate hydrocarbons. Prolonged contact of the unvaporized liquid hydrocarbons with catalyst after discharge into a separation vessel may result in further thermal cracking which tends to favor such end reactions particularly at high velocities. Further, it is essential to such catalytic cracking that hydrocarbon contacting the catalyst be as near vapor as possible because such reaction is primarily a vapor phase reaction.
While it has been proposed heretofore to use misting or fine droplet nozzles in the riser reactor pipe, in general such fine dispersions have been obtained by the use of steam or other vaporizing materials which form a two-phase fluid. A particular problem with such two-phase fluids is that in general they produce a higher pressure drop through the spray nozzles than either fluid phase alone. This is important because pressure drop across the nozzle unit for a given size and a given rate of feed has a significant influence on the size of droplets that can be formed by the nozzle. It is, of course, also undesirable to add additional steam to the hydrocarbon feed. Such added steam must be recovered in the overhead distillation column and generally creates a "sour" water disposal problem, because oxides of sulfur, nitrogen and carbon in the recovered hydrocarbon vapors combine with the water to form acids. In spite of such problems, steam is frequently used primarily because it reduces the hydrocarbon partial pressure and accordingly reduces resistance to vaporization of the feed stream by the catalyst.
In the past various hydrocarbon feed systems having feed nozzles were developed. Unfortunately, none of these provided a hydrocarbon feed system wherein a single liquid stream, with or without steam included therein, is injected into a flowing stream of fluidized catalytic particles and then promptly misted. Such a system and nozzle is disclosed in misted. Such a system and nozzle is disclosed in commonly assigned and U.S. Pat. No. 4,793,913 issued Dec. 27, 1988, the disclosure of which is incorporated herein by reference. In that disclosure the feed is misted by generating a free vortex in the single liquid hydrocarbon stream prior to injection by passing it through a centrifugal acceleration chamber, including vanes, and then releasing the full flow through a sharp or square-edge discharge orifice near the reactor side wall. The orifice is so positioned that the vena contracta (the contraction of the diameter of the cylindrical jet of fluid coming out of the orifice to a cross-section smaller than the orifice) of fluid flowing through the orifice is sufficiently close to the flowing catalyst stream in the riser reactor to maintain its solid-liquid flow form into the catalyst stream. As further disclosed the nozzle orifice is located within the side wall to assure that the liquid stream breaks into a fine mist over a conical pattern well within the catalyst stream. Such mounting assures that the outer surface of the nozzle is not coked by the feed or abraded (with eventual destruction of the metal nozzle) by high velocity catalyst particles made of exceedingly abrasive compounds, such as alumina and/or silica, flowing over the outer surface of the nozzle. The square-edged orifice nozzles have been found highly effective to improve hydrocarbon yields of transportation liquids, such as gasoline, kerosene and jet fuels.
It has subsequently been discovered, however, that such an optimum location of the nozzle orifice and the entire nozzle unit within the riser sidewall carrying the catalyst stream is most effective in a relatively small diameter reactor riser. A more preferred location for relatively large diameter risers is within a protective pipe extending into the riser stream. The protective pipe should be concentric with the nozzle unit for extended life of the nozzle. Such further protection is provided in accordance with the present invention by positioning the nozzle, including its discharge orifice, so that the entire assembly is mounted within an opening in the riser sidewall. Unfortunately, it has been discovered that such a nozzle creates a self-destructive eddy current from the stream of high-velocity particles within the sidewall or a protective pipe which first erodes away the surface of the interior of the opening or protective pipe and then the improved nozzle. It is therefore highly desirable to have some way of locating such a nozzle unit for optimum efficiency while at the same time protecting such a nozzle unit and associated structure from destructive eddy currents. The instant invention provides a solution to that problem by providing the combination of a nozzle unit having a square-edged orifice and a special protective pipe or shroud having an abrasion resistant surface on the inside of the pipe so that it surrounds the flow stream, including the vena contracta from the orifice, along with an improved method of use thereof.